1. Technical Field of the Invention
The present invention relates to a method and base station for allocating pseudo noise (PN) codes on a downward transmission channel which carries spread spectrum signals for code division multiple access (CDMA) from a base station to a plurality of mobile stations.
2. Description of the Prior Art
The spread spectrum signals modulated by the PN codes properly allocated to the mobile stations are multiplexed and transmitted from the base station to the mobile stations. Therefore, the transmission signals are easily synchronized, because the transmission signals are transmitted simultaneously onto the downward direction. Further, the interference between the transmission signals can be suppressed by using the orthogonal code.
However, it difficult to synchronize the signals from one base station with the signals from the other base station. Therefore, the scramble code such as Gold code or a part thereof are used in addition to the orthogonal code.
A conventional transmitter of the base station is shown in FIG. 17. In spread adder unit SPU as shown in FIG. 17, transmission signal to mobile station Si (“i”=1 to n) is multiplied by orthogonal code Co,i in multiplier 001. Then, the outputs from multipliers 001 are added in adder 002. Further, the output from adder 002 is multiplied by scramble code Cs in multiplier 001 in order to obtain combined signal.
Combined signal is modulated in modulator 003 and transmitted from antenna 004 to the mobile stations. When all the base stations employ the transmitter as shown in FIG. 17, the interference between the signals from the base stations are randomized, or in other words, suppressed.
It must be avoided that numerous codes are used unnecessarily, because the number of the orthogonal codes are limited. However, there are several cases of the unnecessary consumption of code in CDMA system.
The code consumption is increased, when a plurality of the base stations transmit the identical signals towards one mobile station in the soft hand-over (SHO).
The code consumption is also increased, when the base station intends to control many mobile stations by cutting off the communication with a mobile station whose signal goes through silent period.
Further, the code consumption is increased, when intercellular interference is weakened due to many obstacles.
Therefore, when the number of the mobile stations is greater than the number of the orthogonal codes in a base station, the base station has to use a plurality of the scramble codes.
A conventional transmitter of the base station which employs “k” scramble codes is shown in FIG. 18. Spread adder unit SPU-i (i=1 to k) outputs combined spread signal Sd-j on the basis of transmission signal Si,nj, orthogonal code Co, nj, and scramble code Cs,i. Here, signal Si,nj is a signal which is transmitted by a “nj”-th mobile station with scramble code “j”. Further, number “nj” is the number of the orthogonal codes generated by j-th scramble code Cs,j. Transmission signal Si,nj may contain control signal which is commonly used. Combined spread signals Sd-i (for i=1 to k) are added in adder 002. The output from adder 002 are modulated by modulator 003. Antenna 004 transmits the output of modulator 003 towards the mobile stations.
Spread adder unit SPU-j outputs combined spread spectrum signal Sd-j on the basis of transmission signal Sj,h. Here, “h” is an ordinal number of orthogonal code Co,h, and Cs,j is a scramble code. Combined spread spectrum signal Sd-1, Sd-2, . . . , and Sd-k are added in adder 002. The output from adder 002 are fed into modulator 003 to be transmitted from antenna 004 towards the mobile stations.
An example of a relation of signal to interference noise ratio (SINR) vs. combined code is shown in FIG. 19. The combined code is expressed, for example, as (2, n2), where “2” is an ordinal number of scramble code and “n2” is an ordinal number of orthogonal code. The SINR as shown in FIG. 19 is low for every combined code, when all the combined codes are used randomly or with equal frequency in use.
When a plurality of the scramble codes are used simultaneously, the mobile station suffers interference due to signals with other scramble codes for other stations. Thus, quality of communication channel is degraded, because the interference between different scramble codes is greater than that between the identical scramble codes.